Why isn't there an

at(At.list(), 12)

that allows to pinpoint on element #12 in a list in Optics? Maybe there's another way to do this that I'm missing?

there's

at

and

index

in the

arrow.optics.dsl

package

Oh ...

index(Index.list(), 0)

that wasn't so obvious. Why not just

at(At.list(), 0)

just like there is for

At.set()

and

At.map()

?

At

and

Index

are quite different,

At

works with a

Lens

while

Index

works with an

Optional

.

At

works quite different than what you're describing for

Map

and

Set

.

At

for a

Set<A>

looks whether an

A

is present, and checking if the

Set

contains

that

A

or it allows you to remove it from the

Set

. (

Lens<Set<A>, Boolean>

).

At

for

Map<K, V>

looks wheter a value is present at

K

, and allows retrieving or removing that value using

Option<V>

. So setting

None

removes it, and setting

Some(V)

updates it. (

Lens<Map<K, V>, Option<V>>

). So that is extremely different from what working over an

Index

does.

Thanks for the explanation! So

index(..)

is saying, if it's there, get/set/transform it, if not, nothing happens (and return none?). Whereas

at(...)

is pinpoint at an element, if it's there, get/modify/remove it, if not, create it? I still don't understand why Arrow wouldn't provide a specialized

index(Index.listBy { it.id }, FooId(1))

for fetching by map key or some property in an element in a Set... But maybe I'm starting to understand why

at(...)

is not implemented for a list (by index at least), because we could get to invalid states like trying to remove an element in the middle of a list (thereby changing all the indices). But it could still be useful to have an

at(At.listBy { it.id }, 1)

where

id

is a property on the elements in the list...